Tunable band pass filter



W. SICHAK ETAL TUNABLE BAND PASS FILTER Filed July 13, 1951 III||||""" A. ,queens/.1cm JR mmy L t4er 15a United States Patent Olice 2,697,209 Patented Dec. 14, 1954 TUNABLE BAND PASS FILTER Application July 13, 1951, Serial No. 236,576 9 Claims. (Cl. 333-73) This invention relates to wave guide structures for microwaves and more particularly to band pass filters of the wave guide type.

Band pass filters of the fixed tuned type for microwaves have been built heretofore and worked satisfactorily at a given frequency. It has been proposed to tune such filters by locating tuning posts or lumped susceptance in each cavity of the lter. Such methods of tuning, however, result in the pass-band response becoming asymmetrical as the filter is timed in either direction away from the design frequency. Thus, lumped susceptance tuning methods limit the filter objectionably to narrow frequency ranges.

One of the objects of this invention is to provide a tunable filter having a band pass characteristic which exhibits symmetrical pass-band response and essentially constant band width throughout the tuning range.

Another object is to provide a tunable filter in which the resonant frequency may be varied while preserving the design characteristics of the filter.

Still another object of the invention is to provide a tunable wave guide lter having a means for varying at least one of the parameters of the wave guide in such a manner that the guide wave length at resonance is substantially constant.

One of the features of the invention is that the constant guide wavelength filter characteristics of the tunable ltei provides foi' easy coupling to transmission lines which remain substantially tuned over the entire frequency range of the filter. Thus, there is substantially no asymmetrical response tending to introduce mismatch to the coupling transmission lines with which the filter is associated. Another feature is that the filter may be tuned by a single control which is easily calibrated.

Briefly, the preferred embodiment of the invention comprises a wave guide whose guide wavelength is changed capacitively by the presence of a dielectric strip adapted to be adjusted at any desired depth in a slot cut in the broad face of the wave guide section. The filter is designed to operate at the highest desired frequency when the strip is completely out of the guide. To tune the filter to a lower desired frequency, the dielectric strip is adjusted in depth of insertion in the wave guide. The dielectric strip is made wide enough so that the guide wavelength, with the strip almost all the Way across the guide at the lowest desired frequency, is the same as the design guide wavelength. Also, rather simple matching transformers are provided which are capable of coupling the sections to transmission lines and other microwave apparatus. A good approximation to the constant bandwidth can be obtained by using halfor quarterwavelength transformers between the constant guide wave vlength filter and the rectangular form of waveguide.

The above mentioned and other features and objects of the invention and the manner of obtaining them will become more apparent by reference to the following de- P scription taken in conjunction with the accompanying drawings, wherein:

Fig. l is a View in perspective of a tunable wave guide filter in accordance with the principles of this invention;

Fig. 2 is a cross-sectional view taken along line 2-2 of Fig. l;

Fig. 3 is a view in side elevation of the filter means for manually tuning the filter;

Fig. 4 shows a resonant frequency response curve for :he filter; and

showing Cil Fig. 5 shows a set of curves showing deviation from resonant frequency.

Referring to Figs. 1 and 2 of the drawing, the illustrated embodiment of the invention is shown to comprise a rectangular wave guide section 1 along which microwave energy may be propagated in the TELO mode. The filter section is provided with a series of capacitive irises in the form of conductive plates 2, 3, 4, 5, 6, and 7 spaced to provide a three-element or cavity filter. irises are arranged in pairs, 2, 3, 4, 5 and 6, 7, each pair being spaced apart a distance equal substantially to slightly more than one-half Wavelength of the resonant frequency. The pairs of irises are spaced apart by a distance equal substantially to three-quarters of a Wavelength of the resonant frequency for which the filter is designed. The upper wall 8 of the wave guide section 1 is provided with a longitudinal slot 9 to l0 of dielectric material such as polystyrene, polyethylene, Telion or other high quality dielectric, which provides a variable capacitance. The strip has transverse slots 11 to accommodate the irises. The ends of the strip are reduced in thickness as indicated at 12 and 13 to form transformer sections to minimize perturbation of the waves at the ends of the lilter. The length of the transformer end portions 12 and 13 are substantially a quarter wavelength long.

In Fig. 3 the filter is shown in a form provided with a manual control for determining the depth of the dielectric strip 10 within the guide section 1. The saine reference characters are used in Figs. l, 2, and 3 to indicate corresponding parts.

The irises in Fig. 3 are shown to be contained within the wave guide 1 and are indicated by the reference characters 2a, 3a, 4a, 5a, 6a, and 7a. The showing of the irises as extending outside the surface of the wave guide in Figs. `1 and 2 merely illustrates better the location of the irises. In a commercial model the irises are contained entirely within the wave guide structure substantially as indicated in Fig. 3.

The wave guide section 1 is provided with a bracket 14 having a threaded boss 15 to receive the threaded shank 6 of a manually adjustable member connected at i8 to the strip 10, whereby the depth posi- 17 is a pointer 21, whereby the the tuned condition of the filter In one example of the filter the dielectric strip was selected of polystyrene a quarter inch thick, for a threecavity filter such as shown in Fig. l. The iilter was designed to resonate at 5050 mc. per second. Each cavity was designed to have a band width of 84 mc. so that the band width of the entire filter would be 65 rnc. In Fig. 4 the resonant frequency is plotted against the depth of the dielectric strip showing that a straight slope 22 is obtained. In Fig. 5 the measured standing wave ratio in depth of the strip and are readily determined.

l frequency is changed. Single knob tuning and the calibration of the tuning dial with a linear frequency- 'scale is also an guide wave length,

. pass band response and essentially constant band width.

principles of our specific example, it is to description is made by a limitation of the scope 3 of our invention as set forth in the objects thereof and iny the accompanying claims.

We claim:

l. A band pass microwave lilter comprising a section of ak waveguide, a` series` of irises 'spaced apart lengthwise of said waveguide section, a strip of dielectric material, said strip havingk slots disposed transversely thereof and in. alignment with respective 'ones of said irises with a length of said strip extending beyond the end irises of said series, and means disposing said strip at least partially within and lengthwise of saidy waveguide section withk portions of said` strip between said irises, said strip d'et'errninirig'` capacitively the guide wavelength in said section.

2. A band pass microwave lter according to claim i, wherein the ends of said strip beyond said end irises are smaller in one cross-sectional dimension than the remaining portion of said strip.

3. A. band pass microwave filter ycomprising a section of a waveguide, ak pair of irises spaced apart lengthwise of said waveguide section, ay strip of dielectric material, and means disposing said strip at least partially within and lengthwise of said waveguide section between said irises, said strip determining capacitively the guide wavelength in said section, and the ends of said dielectric strip being extended beyond said irises, each end having a part which is narrower than the remaining portion of the strip.

4. A band pass microwave filter comprising a length of wave guide, pairs of irises spacedv at intervals along said wave guide, cach pair being spaced apart a distance equal substantially to slightly more than one-half wavelength of the resonant frequency and the pairs of irises being spaced apart by a distance equal to substantially threequarters of a wavelength of said resonant frequency and aV strip of dielectric extending lengthwise of said wave guide and Vhaving portions extending into said wave guide in the spaces between said irises.

5 A band pass microwave iilt'er according to claim 4,. further including means for adjusting the depth that said dielectric portions are disposed within said spaces to determine the resonant frequency of the iilter.

6. A band pass microwave lte'r comprising a section of a waveguide, a 'pair of irises spaced apart lengthwise of said waveguide section, a strip of dielectric material, and means disposing said strip at least partially within and lengthwise of said waveguide section between said irises, said strip determining capacitively the guide wavelength in said section, extending beyond the end-most irises, said ends having a different width for at least part of their length to act as matching transformers.

7. A band pass microwave lter according to claim 6, wherein said end sections of said strip are equal substantially to one-quarter wavelengthy of the mid-resonant frequency.

8. A tunable band pass microwave lter comprising a length of wave guide, a series of irises spaced at transverse points along said wave guide to provide series coupled resonant cavities, said wave guide section having a longitudinal slot in one wall thereof, a dielectric strip disposed in said slot for movement transversely of said wave guide, said strip having slots disposed transversely thereof and in alignment with respective ones of said irises, and means for adjusting the position of said strip tcl) determine the depth said irises are received in said s ots.

9. A tunable band pass microwave iilter according to claim 8 wherein said means includes a bracket carried by said wave guide section, said bracket having a threaded boss, a threaded member. swivelly connected to said strip and threadibly engaged in said boss, whereby turning movement of said member is adapted to move said strip relatively to said guide.

References Cited in the le of this patent UNITED STATES PATENTS OTHER REFERENCES Publication I,"Microwave Transmission Circuits by Ragan, vol. 9 of Radiation Laboratory Series published by McGraw-Hill in 1948, page 494 relied on; pp. 677--706` of interest. (Copy in Div. 69.)

said strip including end sections 

